A theory is a well-substantiated explanation or set of principles that is used to explain a phenomenon or predict outcomes. It is based on a collection of evidence, observations, and experiments, and is often supported by scientific research. Theories are used in various fields such as science, mathematics, social sciences, and philosophy to provide a framework for understanding and interpreting the world around us. They can be revised or replaced as new evidence emerges or as our understanding of a particular subject deepens.